1. Field of the Invention
This invention generally relates to a gas sensor. This invention relates to, for example, a gas sensor located in an exhaust system of an automotive internal combustion engine for measuring a specific-component concentration such as an oxygen concentration in an exhaust gas produced by the engine.
2. Description of the Related Art
The exhaust system of a typical internal combustion engine for an automotive vehicle is provided with a gas sensor to detect an oxygen concentration in an exhaust gas produced by the engine. The gas sensor includes a sensor element. In general, the sensor element has a solid electrolytic member, and a pair of a measurement electrode and a reference electrode extending on surfaces of the solid electrolytic member. The gas sensor outputs an electric signal representing the detected oxygen concentration in the exhaust gas. An engine control system adjusts conditions of the burning of an air-fuel mixture in the engine in response to the output signal of the gas sensor to implement fuel economy and exhaust emission control.
The sensor element within the gas sensor is active only when its temperature is equal to or higher than a given value. It is known to provide a heater in the gas sensor. The sensor element is heated to the given temperature or higher by the heater so that the gas sensor can operate from a moment immediately after the start of the engine.
In the case where high power is fed to the heater, the output signal from the gas sensor tends to contain considerable noise components immediately after the start of the engine. Such noise components cause wrong control of conditions of the burning of an air-fuel mixture in the engine.
Japanese published unexamined patent application 4-157358 discloses an oxygen sensor for detecting an oxygen concentration in an exhaust gas produced by an automotive internal combustion engine. The oxygen sensor includes a solid electrolytic member having a hollow cylindrical shape or a cup-like shape. A pair of electrodes are provided on inner and outer surfaces of the solid electrolytic member, respectively. A heater is disposed in the solid electrolytic member. An end of the heater contacts the inner surface of a bottom of the solid electrolytic member. The oxygen sensor is designed to fall into a correctly operating state in a short time after the start of the activation of the heater. In the oxygen sensor, a ratio Rh/Rs is equal to or greater than 0.8 where Rh denotes the outside diameter of the heater and Rs denotes the inside diameter of the solid electrolytic member. A ratio L/Rh is equal to or smaller than 2 where L denotes the axial length of a heating member in the heater. A ratio D/Rs is equal to or smaller than 0.6 where D denotes the distance between a lower end of the heating member and the inner surface of the bottom of the solid electrolytic member.
Japanese published unexamined patent application 8-122297 discloses an oxygen sensor including a solid electrolytic member having a cup-like shape or a hollow cylindrical shape. Inner and outer electrodes are provided on inner and outer surfaces of the solid electrolytic member, respectively. A heater is disposed in the solid electrolytic member. A high-emissivity layer is provided between the heater and the inner surface of the solid electrolytic member (or the inner electrode). The high-emissivity layer efficiently transmits heat from the heater to the solid electrolytic member.
It is an object of this invention to provide a gas sensor which can output a noise-free signal even at an initial operation stage.
A first aspect of this invention provides a gas sensor comprising a sensor element including a solid electrolytic member, a measurement electrode, and a reference electrode, the measurement electrode being provided on the solid electrolytic member and being exposed to a measurement gas, the reference electrode being provided on the solid electrolytic member and being exposed to a reference gas; a heater for heating the sensor element, wherein a portion of the heater contacts a portion of the sensor element; and means for enabling a temperature of the sensor element to be increased to 300xc2x0 C. by the heater in ten seconds after a start of activation of the heater.
A second aspect of this invention is based on the first aspect thereof, and provides a gas sensor wherein a leak resistance RL between the heater and the sensor element, and an internal resistance Ri of the sensor element have a relation as follows:
LOG(RL/Ri)xe2x89xa72
A third aspect of this invention is based on the first aspect thereof, and provides a gas sensor wherein the reference electrode faces the heater and contains a high-emissivity material.
A fourth aspect of this invention is based on the first aspect thereof, and provides a gas sensor wherein the heater has a blacked surface facing the sensor element.